Video displays, such as are used for television viewing and computer operation, often use kinescopes, picture tubes, or cathode ray tubes (CRTs) as the display device. A picture tube is a vacuum tube which has a phosphorescent display screen and control terminals for directing a focused electron beam toward the screen to generate the desired image. In general, a picture tube requires a relatively high anode or “ultor” voltage to accelerate the electron beam toward the screen, a cathode and a grid which coact for modulating the intensity of the electron beam in accordance with the image to be generated, and a focus electrode to which a focus voltage is applied to cause the electron beam to be focused at the screen. In addition, a picture tube is associated with a deflection arrangement for deflecting the electron beam both vertically and horizontally. The ultor or anode voltage of the picture tube is often regulated in order to reduce voltage changes attributable to interaction between the internal impedance of the ultor voltage source and the varying cathode or beam current required to generate an image. “Static” focus voltage is applied to the focus terminal of the picture tube in order to focus the electron beam at a given location, such as the center of the screen. It is well understood that the value of the “static” focus voltage is desirably a fixed proportion of the ultor voltage. Dynamic focus control is often provided for adjusting the value of the focus voltage applied to the picture tube in accordance with the position of the electron beam, in order to keep the electron beam focused on the screen notwithstanding the changing length of the electron beam path attributable to deflection.
In one prior art arrangement, the focus amplifier includes a pull-up transistor and a pull-down transistor. The pull-down transistor is responsive to an input signal at a frequency related to a deflection frequency for applying the amplified input signal to a capacitive load that includes the focus electrode capacitance. A high voltage source develops a high voltage at the collector of the pull-up transistor. The emitter of the pull-up transistor is coupled to the capacitive load. It may be desirable to generate the base voltage of the pull-up transistor in a manner that reduces power consumption from the high voltage source.
In carrying out an inventive feature, the base voltage of the pull-up transistor is generated in a storage capacitor from a charge stored in the capacitive load. A transfer of charge from the capacitive load to the storage capacitor does not add any significant power dissipation to a drive circuit that generates the base voltage of the pull-up transistor. Thereby, advantageously, power losses are reduced.